A conventional rotary fuel pump includes a cam ring which is angularly adjustable with respect to a pump housing. The cam ring includes a plurality of cam lobes and encircles part of a distributor member which includes pumping plungers which are slidable within respective bores of the distributor member. The pumping plungers have associated respective shoe and roller arrangements, the rollers of which are engagable with the cam surface of the cam ring. In use, fuel is supplied to the bores of the distributor member by a transfer pump, a force due to fuel pressure within the bores serving to urge the plungers in a radially outward direction. The output pressure of the transfer pump (referred to as “transfer pressure”) is controlled so as to be related to the speed of operation of the engine with which the pump is being used. Rotation of the distributor member relative to the cam ring causes the rollers to move relative to the cam ring, engagement between the rollers and the cam lobes thereby causing the plungers to be forced in a radially inward direction to pressurise fuel within the respective bore and causing fuel to be delivered by the pump at relatively high pressure. By altering the angular position of the cam ring by means of an advance arrangement, the timing at which fuel is delivered by the pump can be adjusted.
The advance piston is movable in response to fuel pressure changes within an advance piston control chamber. Fuel pressure within the advance piston control chamber is controlled by means of a servo-valve including a servo-piston which is movable within a further bore provided in the advance piston. The servo-piston has an associated servo control chamber to which fuel is supplied at transfer pressure, the pressure of fuel within the servo control chamber opposing a force due to a servo control spring arranged within a light load control chamber at the opposite end of the servo-piston. If the speed of rotation of the engine increases, resulting in an increase in transfer pressure, fuel pressure within the servo control chamber is increased, thereby applying a force to the servo-piston to oppose the force due to the servo spring. The servo-piston is therefore urged in a direction in which a fill passage is opened to the advance piston control chamber, permitting fuel to flow from the servo control chamber to the advance piston control chamber. As a result, fuel pressure within the advance piston control chamber is increased, increasing the volume of the advance piston control chamber, and the advance piston is caused to move in a direction to advance the timing of fuel delivery.
The pressure of fuel delivered to the servo control chamber is reduced as engine speed is decreased, under which circumstances the servo control spring serves to urge the servo-piston into a position in which a drain passage in communication with the advance piston control chamber is opened to low pressure, thereby reducing fuel pressure in the advance piston control chamber and causing the advance piston to move to a position in which the timing of fuel delivery is retarded.
The drain passage and the fill passage are defined by radially extending drillings provided in the advance piston. The control edges of the drillings at the surface of the advance piston are spaced axially from one another by, typically, around 0.4 mm. A problem can arise if leakage of fuel into and out of the advance piston control chamber causes the advance piston to drift between a first position in which the fill passage is opened to permit fuel flow from the servo control chamber to the advance piston control chamber, and a second position in which the fill passage is closed and the drain passage is opened to permit fuel flow from the advance piston control chamber to low pressure, whilst the servo-piston remains in a fixed position. For example, if the engine timing is retarded such that the servo control piston is in a position in which the advance piston control chamber communicates with the low pressure drain through the drain passage, any fuel leakage into the advance piston control chamber may cause the advance piston to drift to a position in which the drain passage is closed by the servo-piston and the fill passage is opened. In such circumstances, the advance piston is caused to switch from a retard timing state to an advance timing state resulting in an undesirable shift in engine timing. The same problem can arise in the reverse situation if the advance piston is caused to drift from an advance timing position to a retard timing position.
It has also been observed that a problem occurs at the end of each pumping event as the rollers move over the lobes of the cam surface and the pumping plungers start their outward, return stroke within their respective plunger bores. At the point at which the rollers ride over the cam lobe, a significant force is transmitted through the cam ring and the peg to the advance piston, tending to urge the advance piston in a direction to advance timing. As a result, there is an increased fuel pressure within the light load control chamber which serves to urge the servo-piston in the opposite, retard timing direction. In such circumstances, the advance piston and the servo-piston are therefore moving almost exactly 180o out of phase with one another and, as a result, consistent and accurate control of the advance piston is difficult to achieve.